1/*
2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain, as explained at
4 * http://creativecommons.org/publicdomain/zero/1.0/
5 */
6
7package java.util.concurrent;
8
9import java.util.concurrent.locks.AbstractQueuedSynchronizer;
10
11/**
12 * A synchronization aid that allows one or more threads to wait until
13 * a set of operations being performed in other threads completes.
14 *
15 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
16 * The {@link #await await} methods block until the current count reaches
17 * zero due to invocations of the {@link #countDown} method, after which
18 * all waiting threads are released and any subsequent invocations of
19 * {@link #await await} return immediately.  This is a one-shot phenomenon
20 * -- the count cannot be reset.  If you need a version that resets the
21 * count, consider using a {@link CyclicBarrier}.
22 *
23 * <p>A {@code CountDownLatch} is a versatile synchronization tool
24 * and can be used for a number of purposes.  A
25 * {@code CountDownLatch} initialized with a count of one serves as a
26 * simple on/off latch, or gate: all threads invoking {@link #await await}
27 * wait at the gate until it is opened by a thread invoking {@link
28 * #countDown}.  A {@code CountDownLatch} initialized to <em>N</em>
29 * can be used to make one thread wait until <em>N</em> threads have
30 * completed some action, or some action has been completed N times.
31 *
32 * <p>A useful property of a {@code CountDownLatch} is that it
33 * doesn't require that threads calling {@code countDown} wait for
34 * the count to reach zero before proceeding, it simply prevents any
35 * thread from proceeding past an {@link #await await} until all
36 * threads could pass.
37 *
38 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
39 * of worker threads use two countdown latches:
40 * <ul>
41 * <li>The first is a start signal that prevents any worker from proceeding
42 * until the driver is ready for them to proceed;
43 * <li>The second is a completion signal that allows the driver to wait
44 * until all workers have completed.
45 * </ul>
46 *
47 * <pre> {@code
48 * class Driver { // ...
49 *   void main() throws InterruptedException {
50 *     CountDownLatch startSignal = new CountDownLatch(1);
51 *     CountDownLatch doneSignal = new CountDownLatch(N);
52 *
53 *     for (int i = 0; i < N; ++i) // create and start threads
54 *       new Thread(new Worker(startSignal, doneSignal)).start();
55 *
56 *     doSomethingElse();            // don't let run yet
57 *     startSignal.countDown();      // let all threads proceed
58 *     doSomethingElse();
59 *     doneSignal.await();           // wait for all to finish
60 *   }
61 * }
62 *
63 * class Worker implements Runnable {
64 *   private final CountDownLatch startSignal;
65 *   private final CountDownLatch doneSignal;
66 *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
67 *     this.startSignal = startSignal;
68 *     this.doneSignal = doneSignal;
69 *   }
70 *   public void run() {
71 *     try {
72 *       startSignal.await();
73 *       doWork();
74 *       doneSignal.countDown();
75 *     } catch (InterruptedException ex) {} // return;
76 *   }
77 *
78 *   void doWork() { ... }
79 * }}</pre>
80 *
81 * <p>Another typical usage would be to divide a problem into N parts,
82 * describe each part with a Runnable that executes that portion and
83 * counts down on the latch, and queue all the Runnables to an
84 * Executor.  When all sub-parts are complete, the coordinating thread
85 * will be able to pass through await. (When threads must repeatedly
86 * count down in this way, instead use a {@link CyclicBarrier}.)
87 *
88 * <pre> {@code
89 * class Driver2 { // ...
90 *   void main() throws InterruptedException {
91 *     CountDownLatch doneSignal = new CountDownLatch(N);
92 *     Executor e = ...
93 *
94 *     for (int i = 0; i < N; ++i) // create and start threads
95 *       e.execute(new WorkerRunnable(doneSignal, i));
96 *
97 *     doneSignal.await();           // wait for all to finish
98 *   }
99 * }
100 *
101 * class WorkerRunnable implements Runnable {
102 *   private final CountDownLatch doneSignal;
103 *   private final int i;
104 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
105 *     this.doneSignal = doneSignal;
106 *     this.i = i;
107 *   }
108 *   public void run() {
109 *     try {
110 *       doWork(i);
111 *       doneSignal.countDown();
112 *     } catch (InterruptedException ex) {} // return;
113 *   }
114 *
115 *   void doWork() { ... }
116 * }}</pre>
117 *
118 * <p>Memory consistency effects: Until the count reaches
119 * zero, actions in a thread prior to calling
120 * {@code countDown()}
121 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
122 * actions following a successful return from a corresponding
123 * {@code await()} in another thread.
124 *
125 * @since 1.5
126 * @author Doug Lea
127 */
128public class CountDownLatch {
129    /**
130     * Synchronization control For CountDownLatch.
131     * Uses AQS state to represent count.
132     */
133    private static final class Sync extends AbstractQueuedSynchronizer {
134        private static final long serialVersionUID = 4982264981922014374L;
135
136        Sync(int count) {
137            setState(count);
138        }
139
140        int getCount() {
141            return getState();
142        }
143
144        protected int tryAcquireShared(int acquires) {
145            return (getState() == 0) ? 1 : -1;
146        }
147
148        protected boolean tryReleaseShared(int releases) {
149            // Decrement count; signal when transition to zero
150            for (;;) {
151                int c = getState();
152                if (c == 0)
153                    return false;
154                int nextc = c - 1;
155                if (compareAndSetState(c, nextc))
156                    return nextc == 0;
157            }
158        }
159    }
160
161    private final Sync sync;
162
163    /**
164     * Constructs a {@code CountDownLatch} initialized with the given count.
165     *
166     * @param count the number of times {@link #countDown} must be invoked
167     *        before threads can pass through {@link #await}
168     * @throws IllegalArgumentException if {@code count} is negative
169     */
170    public CountDownLatch(int count) {
171        if (count < 0) throw new IllegalArgumentException("count < 0");
172        this.sync = new Sync(count);
173    }
174
175    /**
176     * Causes the current thread to wait until the latch has counted down to
177     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
178     *
179     * <p>If the current count is zero then this method returns immediately.
180     *
181     * <p>If the current count is greater than zero then the current
182     * thread becomes disabled for thread scheduling purposes and lies
183     * dormant until one of two things happen:
184     * <ul>
185     * <li>The count reaches zero due to invocations of the
186     * {@link #countDown} method; or
187     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
188     * the current thread.
189     * </ul>
190     *
191     * <p>If the current thread:
192     * <ul>
193     * <li>has its interrupted status set on entry to this method; or
194     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
195     * </ul>
196     * then {@link InterruptedException} is thrown and the current thread's
197     * interrupted status is cleared.
198     *
199     * @throws InterruptedException if the current thread is interrupted
200     *         while waiting
201     */
202    public void await() throws InterruptedException {
203        sync.acquireSharedInterruptibly(1);
204    }
205
206    /**
207     * Causes the current thread to wait until the latch has counted down to
208     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
209     * or the specified waiting time elapses.
210     *
211     * <p>If the current count is zero then this method returns immediately
212     * with the value {@code true}.
213     *
214     * <p>If the current count is greater than zero then the current
215     * thread becomes disabled for thread scheduling purposes and lies
216     * dormant until one of three things happen:
217     * <ul>
218     * <li>The count reaches zero due to invocations of the
219     * {@link #countDown} method; or
220     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
221     * the current thread; or
222     * <li>The specified waiting time elapses.
223     * </ul>
224     *
225     * <p>If the count reaches zero then the method returns with the
226     * value {@code true}.
227     *
228     * <p>If the current thread:
229     * <ul>
230     * <li>has its interrupted status set on entry to this method; or
231     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
232     * </ul>
233     * then {@link InterruptedException} is thrown and the current thread's
234     * interrupted status is cleared.
235     *
236     * <p>If the specified waiting time elapses then the value {@code false}
237     * is returned.  If the time is less than or equal to zero, the method
238     * will not wait at all.
239     *
240     * @param timeout the maximum time to wait
241     * @param unit the time unit of the {@code timeout} argument
242     * @return {@code true} if the count reached zero and {@code false}
243     *         if the waiting time elapsed before the count reached zero
244     * @throws InterruptedException if the current thread is interrupted
245     *         while waiting
246     */
247    public boolean await(long timeout, TimeUnit unit)
248        throws InterruptedException {
249        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
250    }
251
252    /**
253     * Decrements the count of the latch, releasing all waiting threads if
254     * the count reaches zero.
255     *
256     * <p>If the current count is greater than zero then it is decremented.
257     * If the new count is zero then all waiting threads are re-enabled for
258     * thread scheduling purposes.
259     *
260     * <p>If the current count equals zero then nothing happens.
261     */
262    public void countDown() {
263        sync.releaseShared(1);
264    }
265
266    /**
267     * Returns the current count.
268     *
269     * <p>This method is typically used for debugging and testing purposes.
270     *
271     * @return the current count
272     */
273    public long getCount() {
274        return sync.getCount();
275    }
276
277    /**
278     * Returns a string identifying this latch, as well as its state.
279     * The state, in brackets, includes the String {@code "Count ="}
280     * followed by the current count.
281     *
282     * @return a string identifying this latch, as well as its state
283     */
284    public String toString() {
285        return super.toString() + "[Count = " + sync.getCount() + "]";
286    }
287}
288